Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Gas – vapor or mist contact
Reexamination Certificate
2002-02-15
2004-01-20
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Gas, vapor or mist contact
C162S065000, C162S076000, C162S078000, C162S088000, C162S089000, C162S090000
Reexamination Certificate
active
06679972
ABSTRACT:
This application claims the benefit of International Application Number PCT/EP00/04610, which was published in English on Dec. 7, 2000.
The present invention relates to the bleaching of lignin present in pulp fibers in an aqueous environment such as a pulp suspension. The invention also relates to a process for producing paper or board from pulp containing such bleached lignin.
Lignin is an amorphous polymer which, like a glue, keeps the cellulose fibers together in wood. Some lignin also exist in the fiber wall. In its native state in wood lignin is of light color.
The purpose of chemical pulping, such as kraft pulping, is to dissolve the lignin from the wood matrix with a cooking liquor. During cooking lignin is broken down into smaller units and becomes darker, i.e. increases the light absorption. The dark lignin components, some of which are dissolved and some of which are still in the fiber, give the pulp a dark colour.
In the production of paper or board a large proportion of the lignin is washed off the fibers in one or more washing steps. According to EP 296198 (AGA Aktiebolag) the washing-out of substances which contribute to the chemical oxygen demand (COD) in an alkaline cellulosic pulp is improved if the pH of the wash water is lowered with an acid such as carbon dioxide. According to an article by Östberg, G., 5th International Conference of New Available Technique, The World Pulp and Paper Week, Jun. 4-7, 1996, Stockholm pp. 508-515, the said improved washing results in an increased degree of delignification and a consequent decrease in the consumption of bleaching chemicals. Calcium carbonate is formed in a reaction between calcium and the carbonate ions generated by the carbon dioxide. No increase of calcium carbonate deposition on the equipment was observed.
Sundin, J. et al. in “Precipitation of lignin during pulp washing”, p. 219-227, proceedings, TAPPI Pulping Conference, 1998, discloses that the addition of calcium or magnesium ions to deionized water used for washing a pulp provided washed pulps with a higher kappa number (a common measure of the lignin content) than pulps washed without addition of these ions. There was found to be a correlation between kappa number and the concentration of calcium, magnesium and the like ions in the wash water. The more ions were added, the higher the kappa number.
Sundin et al. also showed that lignin which precipitated during washing with water to which calcium ions had been added was darker than the other residual lignin in the pulp. Pulps washed with water containing calcium ions were significantly darker than those without such ions.
In a recent publication Gustavsson, C. et al. Nordic Pulp and Paper Research Journal Vol 14, No 1, 1999, p 71-81, noted a distinct decrease in the content of calcium when cooking with a high hydroxide ion concentration. An explanation to the phenomenon is said to be that the calcium is bound to dissolved lignin fragments at high hydroxide ion concentration and, consequently, there is a decreased precipitation of calcium carbonate on the pulp fibers.
On the other hand, WO 90/09483 (Mo och Donsjö Aktiebolag) utilizes the tendency of calcium carbonate to precipitate at an alkaline pH for reducing the amount of gypsum (calcium sulphate) precipitation in a paper stock. A precipitation of the calcium carbonate is induced by adding (bi)carbonate ions to the system. The ions may be created in situ by an addition of sodium hydroxide and carbon dioxide.
According to WO 99/00545 (Sunds Defibrator Industries AB) the release of calcium ions into the filtrate of a papermaking system by acidic bleaching conditions below pH 7 cause harmful precipitations on the equipment. The calcium ions are absorbed on the pulp by an addition of carbonate ions and removed with the pulp.
According to U.S. Pat. No. 5,139,613 (Canadian Liquid Air Limited) carbon dioxide is used to acidify a pulp slurry after it has been bleached with hypochlorite or hydrogen peroxide. The souring with CO
2
instead of SO
2
does not destroy the bleaching chemical which allows the residual bleaching chemicals to continue their bleaching effect for a longer time.
According to GB Patent Application 2 008 562 carbon dioxide may be used for increasing the solubility of calcium carbonate and for the hardening of recycled waters used in the treatment of pulp from waste paper.
Carbon dioxide is a gas, which easily dissolves under alkaline conditions, e.g. in water or a pulp suspension forming carbonic acid and bicarbonate and/or carbonate ions according to the reaction:
CO
2
+H
2
O<=>H
2
CO
3
<=>H
+
+HCO
3
−
<=>2H
+
+CO
3
2−
(1)
Water soluble salts of carbonic acid such as sodium bicarbonate NaHCO
3
and sodium carbonate Na
2
CO
3
also provide bicarbonate and carbonate ions in an aqueous environment.
Carbonate ions have the ability of being bound to calcium ions and of creating calcium carbonate, which is a salt with a low solubility at alkaline pH:
CO
3
2−
+Ca
2+
<=>CaCO
3
(aq)<=>CaCO
3
(s) (2)
At a pH below about 8 calcium carbonate starts to dissolve and the concentration of free calcium ions increases. High concentrations of free calcium ions in a papermaking system are known to cause complex coagulation and scaling problems in papermaking.
The present invention, as defined in the appended claims, is based on the realization that carbonic acid and salts thereof, which provide carbonate and bicarbonate ions, can be used for bleaching calcium containing lignin dissolved or present in pulp fibers in an aqueous pulp suspension.
Although not wishing to be bound by any theory, it is believed that the (bi)carbonate ions are capable of trapping calcium present in the system and thus reducing the influence of calcium on the lignin. This, in turn, provides lignin compounds in the pulp which are lighter in colour. The bleached lignin compounds may also be easier to remove from the fibers causing the pulp to have a reduced lignin content. In either case, the paper resulting from the (bi)carbonate bleached pulp will have an increased brightness. Thus, such a pulp requires a reduced amount of other bleaching chemicals to provide the desired brightness.
The carbonic acid is preferably provided by dissolving carbon dioxide gas in the aqueous pulp suspension. The dissolved gas will produce carbonic acid in the water and will, in turn, provide carbonate and/or bicarbonate ions in the suspension. Carbon dioxide may also be added in liquid or solid form to the suspension.
The carbonates/bicarbonates in the aqueous environment will associate with calcium ions in said environment in such a way that the calcium ions become “trapped” in calcium carbonate. This will reduce the amount of calcium available to influence the lignin. The result will show up as an increased brightness and possibly also as a lower kappa number in the pulp:
Ca
2+
- - - ligin
−
+CO
3
2−
<=>lignin
−
+CaCO
3
(3)
For a high kappa pulp intended to be used in its unbleached form, for example in packaging grades such as linerboard and sack paper, the result of carbon dioxide addition and consequent calcium ion removal from lignin is a brighter pulp/paper with very likely also a lower kappa number at the same cooking conditions. This pulp/paper can be used as such as an unbleached product with higher brightness or the product may provide savings in the top layer of bleached fiber and coating added for optical/printing purposes. The lignin bleaching effect can also be utilized for producing a pulp with the same kappa number as before but with less severe cooking conditions, which means increased productivity, steam savings, increased selectivity or savings in cooking chemicals.
For a pulp intended to be bleached, reducing the influence of calcium ions on lignin makes a pulp with a improved bleachability; consuming less bleaching chemicals to a given brightness.
To reach the desired effect, the
AGA Aktiebolag
Alvo Steve
Pearne & Gordon LLP
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